Unfortunately, the loss of subject detail caused by diffraction impacts the entire image uniformly, where loss of subject detail caused by defocus occurs only at the near and far sharps of DoF. Thus, it's more difficult to discern a loss of resolution that impacts the entire print vs. a loss of resolution that occurs only at the nearest or farthest distances in the subject space, unless you make a side-by-side comparison of two prints.

Any print can be made "sharp" by enhancing its acutance (edge sharpness), through the use of tools like unsharp mask in PS, but rendering actual subject detail at resolutions that cause people to gasp in appreciation, requires that you control both defocus and diffraction, to ensure that both the circles of confusion caused by defocus -and- diffraction's Airy disks are kept small enough, after enlargement, to satisfy your desired print resolution (which should have been chosen with consideration for an anticipated viewing distance).

Remember that if the DoF calculator calls for an f-Number that you can't use (because the corresponding shutter speed would be too slow, or because that f-Number is larger than the f-Number at which diffraction will become visible, or because your lens simply doesn't offer that f-Number), all you have to do is....

Back away from the nearest subject until you're far enough away to use a viable f-Number (you'll have to recalculate DoF for this new range of subject distances).

-OR-

Go to a shorter focal length without moving the camera (you'll have to recalculate DoF for this new focal length).

-OR-

Resign yourself to making a smaller print: Reducing the enlargement factor by 1.414x (making a 10-inch print instead of a 14-inch print) allows you to open up one stop and get the same apparent resolution. Reducing the enlargement factor by 2x (making a 7-inch print instead of a 14-inch print) allows you to open up two stops and get the same apparent resolution).

-OR-

Resign yourself to hanging the print in a location (over a piano?) where people can't examine it at your originally anticipated viewing distance: Increasing the viewing distance by a factor of 1.414x allows you to open up one stop. Increasing the viewing distance by a factor of 2x allows you to open up two stops. (This last solution is difficult to enforce, so it's not practical.)

Sometimes, in the interest of maintaining your intended composition (as defined by your original choice of camera position and focal length), it's best to just go for a smaller print (an enlargement factor lower than that specified when you calculated the Circle of Confusion diameter used to produce your DoF calculators) rather than backing away from the nearest subject or selecting a shorter focal length - both of which can change the composition drastically. But you've got to remember that you made this choice, in the field, to go with a smaller print. Don't cop out later and produce the full sized print only to suffer visible degradation caused by defocus and/or diffraction. Stay the course.

Lastly, please realize that there are many factors other than available pixel count, defocus and diffraction that can prevent you from achieving a "desired" final image resolution. What I've written here only describes an approach to controlling defocus and diffraction, with no attention given to lens resolving power at various apertures, the smearing caused by subject or camera motion at inadequate shutter speeds, film resolution and in-camera flatness (for those who are still shooting film), etc. Defocus and diffraction are, however, among the most controllable of factors affecting final image resolution, if you're willing to exercise that control instead of just rolling the dice every time you make an exposure.

For more on how I approach landscape photography (only one of many ways to boil the proverbial egg), read these five posts (or the entire thread) from the Canon Digital Photography Forum: